1. Field of the Invention
The present invention relates to a transparent electroconductive layered structure, organic electroluminescent device using the same, and method for producing the same layered structure and device, more specifically a transparent electroconductive layered structure as a constituent component for an organic electroluminescent device (hereinafter referred to as organic EL device), which is used as a light source (e.g., liquid-crystal device's back light) or display, organic EL device using the same layered structure, and methods for producing the same layered structure and organic EL device.
2. Description of the Prior Art
An electroluminescent device is self-luminous, unlike a liquid-crystal device, and an inorganic electroluminescent device (hereinafter referred to as inorganic EL device) with an inorganic fluorescent material serving as the light-emitting material has been used as a component for displays. An inorganic EL device, however, involves several disadvantages, which have limited its applicable areas. For example, it can emit limited types of colors, and needs a high operating voltage.
Recently, an organic EL device of two-layered structure with an organic light-emitting layer and charge-transporting compound layer has been proposed (as disclosed by, e.g., JP-A-59-194393). It can work at a low voltage to realize high luminance, and easily produce varying emitted light wavelengths. Therefore, it has been extensively studied for commercialization, because of these advantages.
An organic light-emitting layer for an organic EL device has been mainly produced by vacuum evaporation of a low-molecular-weight light-emitting material (refer to JP-A-59-194393) or coating a substrate with a high-molecular-weight light-emitting material or precursor therefor (refer to, e.g., JP-A-03-244630 or JP-A-10-092577), the latter being more noted because of the simpler organic EL device production process and potential for reducing the production cost.
Some of the high-molecular-weight light-emitting materials proposed so far for forming an organic, light-emitting layer include poly(p-phenylene vinylene) as a conjugated high-molecular-weight compound produced by polymerization of the solvent-soluble precursor, in which it is spread, dried and heat-treated at high temperature (refer to JP-A-10-092577), and another conjugated, high-molecular-weight compound soluble in a solvent and needs no heat-treatment at high temperature (refer to JP-A-03-244630).
Moreover, JP-A-2000-514590, for example, proposes a hole-injection layer of electroconductive high-molecular-weight compound, e.g., polythiophene derivative, formed between an anode and light-emitting layer, to improve light-emitting efficiency and durability of the organic EL device in which it is used.
Of the layers which constitute the above organic EL device, the organic light-emitting layer and hole-injection layer are produced by a coating method in which a high-molecular-weight light-emitting compound or electroconductive high-molecular-weight compound is spread and dried. On the other hand, a transparent electroconductive film as the anode is made of an electroconductive oxide, e.g., indium tin oxide (ITO) or antimony tin oxide (ATO), and formed by a physical method, e.g., sputtering.
However, the physical method for producing the transparent electroconductive film is not preferable costwise, because it needs a large-size production system and a film-making apparatus operating under a vacuum. Another disadvantage involved in the physical method is need for heat-treatment of the substrate while the film is being formed thereon. This excludes a plastic for the substrate, because of its low heat resistance.
Therefore, several proposals have been made to form a transparent electroconductive film more simply at lower temperature using a coating liquid dispersed with fine electroconductive particles in a solvent to be spread on a substrate, dried and, as required, heat-treated.
For example, JP-A-4-26768 proposes a coating liquid dispersed with fine ITO particles as the electroconductive particles to form a transparent electroconductive film. JP-A-2000-268639 proposes fine electroconductive particles of a noble metal, e.g., gold or silver, dispersed in a coating liquid to form a transparent electroconductive film of still improved film characteristics, e.g., resistance and transmittance.
These methods, in which a coating liquid is used to form a transparent electroconductive film, involve problems when applied to preparation of an anode for the above-described organic EL device. The transparent electroconductive anode prepared using the coating liquid for forming a transparent electroconductive film invariably has a rougher surface than that prepared by the physical method, because of the presence of fine particles dispersed in the coating liquid. Moreover, the coating liquid contains small quantities of coarse electroconductive particles, which may be secondary particles of agglomerated fine particles. These particles may cause uneven spreading while the coating liquid is spread or dried, and eventually notable irregularities on the film surface.
The irregularities or projections of the transparent electroconductive anode layer, when excessively grow, may cause problems, e.g., electrical short circuit between the transparent electroconductive anode layer and cathode layer, failure of light emission by the organic EL device, notably deteriorated light-emitting efficiency, or the like.
It is therefore difficult to use a transparent electroconductive film prepared by a coating method as the transparent electroconductive film for an organic EL device.